After discussing water hardness and pH in my nutrition class at college, I thought I'd post a little info for new fish owners on what pH is and how it correlates to water hardness.
There are two types of water that you can get when you turn on your facet, hard water, the water that leaves terrible calcium deposits on anything and everything, and soft water, the water that feels good on your skin when you take a shower. What makes these two types of water different? Minerals.
Soft water has an abundance of sodium molecules while hard water is loaded with calcium and magnesium.
Usually, if you have soft water, you have lower pH, meaning it's MORE acidic. (The lower the number, the more acidic) And hard water has higher pH.
If you use water softeners in your home, then that generalization is not always true. By adding sodium to your water, you soften it, but the calcium and magnesium don't just disappear. So it's possible for soft water to still have high pH.
When setting up a fish tank, it's important to know the pH recommened for the species. Don't fret too much over being exact, most fish can and will adapt to the pH if it remains constant, adding chemicals or even natural solutions like IAL or driftwood to lower pH or corals, shells, etc to raise pH, can be worse than just leaving your water where it is. (not that these things can't be used as decorations, but don't rely on them to change your pH)
Before you set up your tank, be sure to check the pH right out of the facet, sometimes the pH can be affected by the cycling process and can give you an innaccurate idea of your pH.
Many older homes have older pipes and as those pipes age, the water can become harder due to minerals that leach into it and thus, the pH can also go up. If you notice a high pH, try going outside and testing water from a hose or even rainwater, most of the time, the water will have a lower pH. Personally, water from my tap comes out at 8.6 pH, but water from my garden hose is 7.2, so testing more than one source is definitely important if it's an option for you.